In a pressurized water nuclear reactor plant, a primary coolant loop and secondary coolant loop are used to produce steam for the production of electricity. In the primary coolant loop, a pressurized fluid is passed through a nuclear reactor and, after being heated, through a line which contains a pressurizer, to a steam generator. The heated fluid enters the primary side of the steam generator which is divided into an inlet section and an outlet section by a divider plate. A tub sheet divides the steam generator into the primary side and a second side, which tube sheet has an array of holes having U-shaped heat transfer tubes inserted therein, which communicate between the inlet section and outlet section of the primary side of the steam generator. In operation, the heat pressurized fluid passes through the U-shaped heat transfer tubes and is discharged from the outlet section of the primary side of the steam generator to a line, containing a primary coolant pump, back to the reactor in a continuous closed loop. Secondary coolant is passed through the secondary side of the steam generator where it is converted into steam by heat released by the primary coolant passing through the U-shaped heat transfer tubes, which steam is used to drive a turbine to produce electricity.
The reactor coolant pressure is controlled by a pressure control system containing a pressurizer, which is a vertical, cylindrical vessel with hemispherical top and bottom heads, wherein water and steam are maintained in equilibrium by electrical heaters and water sprays. Steam can be formed by activating the heaters to increase the pressure in the primary coolant loop, or condensed by the water sprays to reduce the pressure. Power operated relief valves and spring loaded safety valves are connected to the pressurizer and discharge to a pressurizer relief tank, where steam from the pressurizer is condensed and cooled by mixing with water.
The pressure control system for the primary loop thus includes the pressurizer and the associated sprays, heaters, power operated relief valves, safety valves, relief tank, and surge lines. This equipment is designed to accommodate changes in system volume and to limit changes in system pressure due to reactor coolant loop temperature variations during all modes of plant operation.
To reduce the problem of leakage through the valve seats, a water seal is maintained upstream of each valve seat of the pressurizer valves. The pipes connecting the pressurizer nozzles to their respective valves are shaped in the form of a loop seal. If the pressurizer pressure exceeds the set pressure of the valves, they will open, and the water from the loop seal will discharge during the accumulation period.
The power operated relief valves and safety valves, which must close a steam line after the pressure drops below a desired trip threshold, do not fully seal so long as steam continues to pass over the valve seat. Conventionally, the water seal in the loop seal associated with each valve is formed by condensation of steam in the area of the loop seal. During such condensation, steam continues to bleed through the valve creating an upstream pressure (or source) stabilization problem, and this high pressure jet bleeding of steam threatens the valve integrity by rapidly eroding the valve seat. Thus, until a replacement water seal for the valve seat is formed, problems exist.
It is an object of the present invention to provide an improved pressure control system for a pressurized water nuclear reactor system which results in a decrease in the response time of closing of the power operated relief valves and safety valves associated with the pressurizer.
It is another object of the present invention to provide an improved pressure control system for a pressurized water nuclear reactor system that enhances the life and reliability of power operated relief valves and safety valves associated with the pressurizer.
It is a further object of the present invention to provide a method of quickly closing and sealing, by means of a water seal, the power operated relief valves and safety valves associated with the pressurizer of a pressurized water nuclear reactor plant.